Anatomy and Physiology of Animals/The Gut and Digestion - Wikibooks, open books for an open world Anatomy and Physiology of Animals/The Gut and Digestion From Wikibooks, open books for an open world original image by vnysia cc by Contents After completing this section, you should know: what is meant by the terms: ingestion, digestion, absorption, assimilation, egestion, peristalsis and chyme the characteristics, advantages and disadvantages of a herbivorous, carnivorous and omnivorous diet the 4 main functions of the gut the parts of the gut in the order in which the food passes down it The Gut And Digestion[ edit ] Plant cells are made of organic molecules using energy from the sun. This process is called photosynthesis. Animals rely on these ready-made organic molecules to supply them with their food. Some animals (herbivores) eat plants; some (carnivores) eat the herbivores. Herbivores[ edit ] Herbivores eat plant material. While no animal produces the digestive enzymes to break down the large cellulose molecules in the plant cell walls, micro-organisms' like bacteria, on the other hand, can break them down.

Therefore herbivores employ micro-organisms to do the job for them. There are three types of herbivore: The first, ruminants like cattle, sheep and goats, house these bacteria in a special compartment in the enlarged stomach called the rumen. The second group has an enlarged large intestine and caecum, called a functional caecum, occupied by cellulose digesting micro-organisms. These non-ruminant herbivores include the horse, rabbit and rat. Humans also have a cecum and can be classified as the third type of herbivorous class, along with orangutans and gorillas. Plants are a primary pure and good source of nutrients, however they are digested very easily and therefore herbivores have to eat large quantities of food to obtain all they require. Herbivores like cows, horses and rabbits typically spend much of their day feeding. To give the micro-organisms access to the cellulose molecules, the plant cell walls need to be broken down. This is why herbivores have teeth that are adapted to crush and grind. Their guts also tend to be lengthy and the food takes a long time to pass through it. Eating plants have other advantages.

Plants are immobile so herbivores normally have to spend little energy collecting them. This contrasts with another main group of animals - the carnivores that often have to chase their prey. Carnivores[ edit ] Carnivorous animals like those in the cat and dog families, polar bears, seals, crocodiles and birds of prey catch and eat other animals. They often have to use large amounts of energy finding, stalking, catching and killing their prey. However, they are rewarded by the fact that meat provides a very concentrated source of nutrients. Carnivores in the wild therefore tend to eat distinct meals often with long and irregular intervals between them. Time after feeding is spent digesting and absorbing the food. The guts of carnivores are usually shorter and less complex than those of herbivores because meat is easier to digest than plant material. Carnivores usually have teeth that are specialised for dealing with flesh, gristle and bone. They have sleek bodies, strong, sharp claws and keen senses of smell, hearing and sight. They are also often cunning, alert and have an aggressive nature. Omnivores[ edit ] Many animals feed on both animal and vegetable material – they are omnivorous.

Most primates are herbivorous but a few, such as chimpanzees and humans, belong to this category as do pigs and rats. Their food is diverse, ranging from plant material to animals they have either killed themselves or scavenged from other carnivores. Omnivores lack the specialised teeth and guts of carnivores and herbivores but are often highly intelligent and adaptable reflecting their varied diet. Treatment Of Food[ edit ] Whether an animal eats plants or flesh, the carbohydrates, fats and proteins in the food it eats are generally giant molecules (see chapter 1). These need to be split up into smaller ones before they can pass into the blood and enter the cells to be used for energy or to make new cell constituents. For example: The Gut[ edit ] The digestive tract, alimentary canal or gut is a hollow tube stretching from the mouth to the anus. It is the organ system concerned with the treatment of foods. At the mouth the large food molecules are taken into the gut - this is called ingestion. They must then be broken down into smaller ones by digestive enzymes - digestion, before they can be taken from the gut into the blood stream - absorption. The cells of the body can then use these small molecules - assimilation.

The indigestible waste products are eliminated from the body by the act of egestion (see diagram 11.1). Diagram 11.1 - From ingestion to egestion The 4 major functions of the gut are: 1. Transporting the food; 2. Processing the food physically by breaking it up (chewing), mixing, adding fluid etc. 3. Processing the food chemically by adding digestive enzymes to split large food molecules into smaller ones. 4. Absorbing these small molecules into the blood stream so the body can use them. The regions of a typical mammals gut (for example a cat or dog) are shown in diagram 11.2. Diagram 11.2 - A typical mammalian gut The food that enters the mouth passes to the oesophagus, then to the stomach, small intestine, cecum, large intestine, rectum and finally undigested material exits at the anus. The liver and pancreas produce secretions that aid digestion and the gall bladder stores bile. Herbivores have an appendix which they use for the digestion of cellulose.

Carnivores have an appendix but is not of any function anymore due to the fact that their diet is not based on cellulose anymore. Mouth[ edit ] The mouth takes food into the body. The lips hold the food inside the mouth during chewing and allow the baby animal to suck on its mother’s teat. In elephants the lips (and nose) have developed into the trunk which is the main food collecting tool. Some mammals, e.g. hamsters, have stretchy cheek pouches that they use to carry food or material to make their nests. The sight or smell of food and its presence in the mouth stimulates the salivary glands to secrete saliva. There are four pairs of these glands in cats and dogs (see diagram 11.3). The fluid they produce moistens and softens the food making it easier to swallow. It also contains the enzyme, salivary amylase, which starts the digestion of starch. The tongue moves food around the mouth and rolls it into a ball for swallowing. Taste buds are located on the tongue and in dogs and cats it is covered with spiny projections used for grooming and lapping. The cow’s tongue is prehensile and wraps around grass to graze it.

Swallowing is a complex reflex involving 25 different muscles. It pushes food into the oesophagus and at the same time a small flap of tissue called the epiglottis closes off the windpipe so food doesn’t go ‘down the wrong way’ and choke the animal (see diagram 11.4). Diagram 11.3 - Salivary glands Teeth[ edit ] Teeth seize, tear and grind food. They are inserted into sockets in the bone and consist of a crown above the gum and root below. The crown is covered with a layer of enamel, the hardest substance in the body. Below this is the dentine, a softer but tough and shock resistant material. At the centre of the tooth is a space filled with pulp which contains blood vessels and nerves. The tooth is cemented into the socket and in most teeth the tip of the root is quite narrow with a small opening for the blood vessels and nerves (see diagram 11.5). In teeth that grow continuously, like the incisors of rodents, the opening remains large and these teeth are called open rooted teeth. Mammals have 2 distinct sets of teeth. The first the milk teeth are replaced by the permanent teeth.

Diagram 11.5 - Structure of a tooth Types Of Teeth[ edit ] All the teeth of fish and reptiles are similar but mammals usually have four different types of teeth. The incisors are the chisel-shaped ‘biting off’ teeth at the front of the mouth. In rodents and rabbits the incisors never stop growing (open-rooted teeth). They must be worn or ground down continuously by gnawing. They have hard enamel on one surface only so they wear unevenly and maintain their sharp cutting edge. The largest incisors in the animal kingdom are found in elephants, for tusks are actually giant incisors. Sloths have no incisors at all, and sheep have no incisors in the upper jaw (see diagram 11.6). Instead there is a horny pad against which the bottom incisors cut. The canines or ‘wolf-teeth’ are long, cone-shaped teeth situated just behind the incisors. They are particularly well developed in the dog and cat families where they are used to hold, stab and kill the prey (see diagram 11.7).

The tusks of boars and walruses are large canines while rodents and herbivores like sheep have no (or reduced) canines. In these animals the space where the canines would normally be is called the diastema. In rodents like the rat and beaver it allows the debris from gnawing to be expelled easily. The cheek teeth or premolars and molars crush and grind the food. They are particularly well developed in herbivores where they have complex ridges that form broad grinding surfaces (see diagram 11.6). These are created from alternating bands of hard enamel and softer dentine that wear at different rates. In carnivores the premolars and molars slice against each other like scissors and are called carnassial teeth see diagram 11.7). They are used for shearing flesh and bone. Dental Formula[ edit ] The numbers of the different kinds of teeth can be expressed in a dental formula. This gives the numbers of incisors, canines, premolars and molars in one half of the mouth.

The numbers of these four types of teeth in the left or right half of the upper jaw are written above a horizontal line and the four types of teeth in the right or left half of the lower jaw are written below it. Thus the dental formula for the sheep is: 0.0.3.3 3.1.3.3 It indicates that in the upper right (or left) half of the jaw there are no incisors or canines (i.e. there is a diastema), three premolars and three molars. In the lower right (or left) half of the jaw are three incisors, one canine, three premolars and three molars (see diagram 11.6). Diagram 11.6 - A sheep’s skull The dental formula for a dog is: 3.1.4.2 3.1.4.3 The formula indicates that in the right (or left) half of the upper jaw there are three incisors, one canine, four premolars and two molars. In the right (or left) half of the lower jaw there are three incisors, one canine, four premolars and three molars (see diagram 11.7).

Diagram 11.7 - A dog’s skull Oesophagus[ edit ] The oesophagus transports food to the stomach. Food is moved along the oesophagus, as it is along the small and large intestines, by contraction of the smooth muscles in the walls that push the food along rather like toothpaste along a tube. This movement is called peristalsis (see diagram 11.8). Diagram 11.8 - Peristalsis Stomach[ edit ] The stomach stores and mixes the food. Glands in the wall secrete gastric juice that contains enzymes to digest protein and fats as well as hydrochloric acid to make the contents very acidic. The walls of the stomach are very muscular and churn and mix the food with the gastric juice to form a watery mixture called chyme (pronounced kime). Rings of muscle called sphincters at the entrance and exit to the stomach control the movement of food into and out of it (see diagram 11.9). Diagram 11.9 - The stomach Small Intestine[ edit ] Most of the breakdown of the large food molecules and absorption of the smaller molecules take place in the long and narrow small intestine.

The total length varies but it is about 6.5 metres in humans, 21 metres in the horse, 40 metres in the ox and over 150 metres in the blue whale. It is divided into 3 sections: the duodenum (after the stomach), jejunum and ileum. The duodenum receives 3 different secretions: 1) Bile from the liver; 2) Pancreatic juice from the pancreas and 3) Intestinal juice from glands in the intestinal wall. These complete the digestion of starch, fats and protein. The products of digestion are absorbed into the blood and lymphatic system through the wall of the intestine, which is lined with tiny finger-like projections called villi that increase the surface area for more efficient absorption (see diagram 11.10). Diagram 11.10 - The wall of the small intestine showing villi The Rumen[ edit ] In ruminant herbivores like cows, sheep and antelopes the stomach is highly modified to act as a “fermentation vat”. It is divided into four parts. The largest part is called the rumen.

In the cow it occupies the entire left half of the abdominal cavity and can hold up to 270 litres. The reticulum is much smaller and has a honeycomb of raised folds on its inner surface. In the camel the reticulum is further modified to store water. The next part is called the omasum with a folded inner surface. Camels have no omasum. The final compartment is called the abomasum. This is the ‘true’ stomach where muscular walls churn the food and gastric juice is secreted (see diagram 11.11). Diagram 11.11 - The rumen Ruminants swallow the grass they graze almost without chewing and it passes down the oesophagus to the rumen and reticulum. Here liquid is added and the muscular walls churn the food. These chambers provide the main fermentation vat of the ruminant stomach. Here bacteria and single-celled animals start to act on the cellulose plant cell walls. These organisms break down the cellulose to smaller molecules that are absorbed to provide the cow or sheep with energy. In the process, the gases methane and carbon dioxide are produced. These cause the “burps” you may hear cows and sheep making.

Not only do the micro-organisms break down the cellulose but they also produce the vitamins E, B and K for use by the animal. Their digested bodies provide the ruminant with the majority of its protein requirements. In the wild grazing is a dangerous activity as it exposes the herbivore to predators. They crop the grass as quickly as possible and then when the animal is in a safer place the food in the rumen can be regurgitated to be chewed at the animal’s leisure. This is ‘chewing the cud’ or rumination. The finely ground food may be returned to the rumen for further work by the microorganisms or, if the particles are small enough, it will pass down a special groove in the wall of the oesophagus straight into the omasum. Here the contents are kneaded and water is absorbed before they pass to the abomasum. The abomasum acts as a “proper” stomach and gastric juice is secreted to digest the protein. Large Intestine[ edit ] The large intestine consists of the caecum, colon and rectum.

The chyme from the small intestine that enters the colon consists mainly of water and undigested material such as cellulose (fibre or roughage). In omnivores like the pig and humans the main function of the colon is absorption of water to give solid faeces. Bacteria in this part of the gut produce vitamins B and K. The caecum, which forms a dead-end pouch where the small intestine joins the large intestine, is small in pigs and humans and helps water absorption. However, in rabbits, rodents and horses, the caecum is very large and called the functional caecum. It is here that cellulose is digested by micro-organisms. The appendix, a narrow dead end tube at the end of the caecum, is particularly large in primates but seems to have no digestive function. Functional Caecum[ edit ] The caecum in the rabbit, rat and guinea pig is greatly enlarged to provide a “fermentation vat” for micro-organisms to break down the cellulose plant cell walls. This is called a functional caecum (see diagram 11.12). In the horse both the caecum and the colon are enlarged.

As in the rumen, the large cellulose molecules are broken down to smaller molecules that can be absorbed. However, the position of the functional caecum after the main areas of digestion and absorption, means it is potentially less effective than the rumen. This means that the small molecules that are produced there can not be absorbed by the gut but pass out in the faeces. The rabbit and rodents (and foals) solve this problem by eating their own faeces so that they pass through the gut a second time and the products of cellulose digestion can be absorbed in the small intestine. Rabbits produce two kinds of faeces. Softer night-time faeces are eaten directly from the anus and the harder pellets you are probably familiar with, that have passed through the gut twice. Diagram 11.12 - The gut of a rabbit The Gut Of Birds[ edit ] Birds’ guts have important differences from mammals’ guts. Most obviously, birds have a beak instead of teeth. Beaks are much lighter than teeth and are an adaptation for flight. Imagine a bird trying to take off and fly with a whole set of teeth in its head!

At the base of the oesophagus birds have a bag-like structure called a crop. In many birds the crop stores food before it enters the stomach, while in pigeons and doves glands in the crop secretes a special fluid called crop-milk which parent birds regurgitate to feed their young. The stomach is also modified and consists of two compartments. The first is the true stomach with muscular walls and enzyme secreting glands. The second compartment is the gizzard. In seed eating birds this has very muscular walls and contains pebbles swallowed by the bird to help grind the food. This is the reason why you must always supply a caged bird with grit. In birds of prey like the falcon the walls of the gizzard are much thinner and expand to accommodate large meals (see diagram 11.13). Diagram 11.13 - The stomach and small intestine of a hen Digestion[ edit ] During digestion the large food molecules are broken down into smaller molecules by enzymes. The three most important groups of enzymes secreted into the gut are: Amylases that split carbohydrates like starch and glycogen into monosaccharides like glucose. Proteases that split proteins into amino acids.

Lipases that split lipids or fats into fatty acids and glycerol. Glands produce various secretions which mix with the food as it passes along the gut. These secretions include: Saliva secreted into the mouth from several pairs of salivary glands (see diagram 11.3). Saliva consists mainly of water but contains salts, mucous and salivary amylase. The function of saliva is to lubricate food as it is chewed and swallowed and salivary amylase begins the digestion of starch. Gastric juice secreted into the stomach from glands in its walls. Gastric juice contains pepsin that breaks down protein and hydrochloric acid to produce the acidic conditions under which this enzyme works best. In baby animals rennin to digest milk is also produced in the stomach. Bile produced by the liver. It is stored in the gall bladder and secreted into the duodenum via the bile duct (see diagram 11.14). (Note that the horse, deer, parrot and rat have no gall bladder). Bile is not a digestive enzyme.

Its function is to break up large globules of fat into smaller ones so the fat splitting enzymes can gain access the fat molecules. Diagram 11.14 - The liver, gall bladder and pancreas Pancreatic juice[ edit ] The pancreas is a gland located near the beginning of the duodenum (see diagram 11.14). In most animals it is large and easily seen but in rodents and rabbits it lies within the membrane linking the loops of the intestine (the mesentery) and is quite difficult to find. Pancreatic juice is produced in the pancreas. It flows into the duodenum and contains amylase for digesting starch, lipase for digesting fats and protease for digesting proteins. Intestinal juice[ edit ] Intestinal juice is produced by glands in the lining of the small intestine. It contains enzymes for digesting disaccharides and proteins as well as mucus and salts to make the contents of the small intestine more alkaline so the enzymes can work. Absorption[ edit ] The small molecules produced by digestion are absorbed into the villi of the wall of the small intestine. The tiny finger-like projections of the villi increase the surface area for absorption.

Glucose and amino acids pass directly through the wall into the blood stream by diffusion or active transport. Fatty acids and glycerol enter vessels of the lymphatic system (lacteals) that run up the centre of each villus. The Liver[ edit ] The liver is situated in the abdominal cavity adjacent to the diaphragm (see diagrams 2 and 14). It is the largest single organ of the body and has over 100 known functions. Its most important digestive functions are: the production of bile to help the digestion of fats (described above) and the control of blood sugar levels Glucose is absorbed into the capillaries of the villi of the intestine. The blood stream takes it directly to the liver via a blood vessel known as the hepatic portal vessel or vein (see diagram 11.15). The liver converts this glucose into glycogen which it stores. When glucose levels are low the liver can convert the glycogen back into glucose. It releases this back into the blood to keep the level of glucose constant. The hormone insulin, produced by special cells in the pancreas, controls this process.

Diagram 11.15 - The control of blood glucose by the liver Other functions of the liver include: 3. making vitamin A, 4. making the proteins that are found in the blood plasma (albumin, globulin and fibrinogen), 5. storing iron, 6. removing toxic substances like alcohol and poisons from the blood and converting them to safer substances, 7. producing heat to help maintain the temperature of the body. Diagram 11.16 - Summary of the main functions of the different regions of the gut Summary[ edit ] The gut breaks down plant and animal materials into nutrients that can be used by animals’ bodies. Plant material is more difficult to break down than animal tissue. The gut of herbivores is therefore longer and more complex than that of carnivores. Herbivores usually have a compartment (the rumen or functional caecum) housing micro-organisms to break down the cellulose wall of plants. Chewing by the teeth begins the food processing. There are 4 main types of teeth: incisors, canines, premolars and molars.

In dogs and cats the premolars and molars are adapted to slice against each other and are called carnassial teeth. Saliva is secreted in the mouth. It lubricates the food for swallowing and contains an enzyme to break down starch. Chewed food is swallowed and passes down the oesophagus by waves of contraction of the wall called peristalsis. The food passes to the stomach where it is churned and mixed with acidic gastric juice that begins the digestion of protein. The resulting chyme passes down the small intestine where enzymes that digest fats, proteins and carbohydrates are secreted. Bile produced by the liver is also secreted here. It helps in the breakdown of fats. Villi provide the large surface area necessary for the absorption of the products of digestion. In the colon and caecum water is absorbed and micro organisms produce some vitamin B and K. In rabbits, horses and rodents the caecum is enlarged as a functional caecum and micro-organisms break down cellulose cell walls to simpler carbohydrates. Waste products exit the body via the rectum and anus. The pancreas produces pancreatic juice that contains many of the enzymes secreted into the small intestine.

In addition to producing bile the liver regulates blood sugar levels by converting glucose absorbed by the villi into glycogen and storing it. The liver also removes toxic substances from the blood, stores iron, makes vitamin A and produces heat. Worksheet[ edit ] Use the Digestive System Worksheet to help you learn the different parts of the digestive system and their functions. Test Yourself[ edit ] Then work through the Test Yourself below to see if you have understood and remembered what you learned. 1. Name the four different kinds of teeth 2. Give 2 facts about how the teeth of cats and dogs are adapted for a carnivorous diet: 1. 3. What does saliva do to the food? 4. What is peristalsis? 5. What happens to the food in the stomach? 6. What is chyme? 7. Where does the chyme go after leaving the stomach? 8. What are villi and what do they do? 9. What happens in the small intestine? 10. Where is the pancreas and what does it do? 11. How does the caecum of rabbits differ from that of cats? 12. How does the liver help control the glucose levels in the blood? 13.

Give 2 other functions of the liver: 1. Vestigiality of the human appendix References Introduction Many biological structures can be considered vestiges given our current evolutionary knowledge of comparative anatomy and phylogenetics. In evolutionary discussions the human vermiform appendix is one of the most commonly cited vestigial structures, and one of the most disputed. Evolutionary vestiges are, technically, any diminished structure that previously had a greater physiological significance in an ancestor than at present. Independently of evolutionary theory, a vestige can also be defined typologically as a reduced and rudimentary structure compared to the same homologous structure in other organisms, as one that lacks the complex functions usually found for that structure in other organisms (see, e.g. Geoffroy 1798 ). Classic examples of vestiges are the wings of the ostrich and the eyes of blind cavefish. These vestigial structures may have functions of some sort. Nevertheless, what matters is that rudimentary ostrich wings are useless as normal flying wings, and that rudimentary cavefish eyes are useless as normal sighted eyes. Vestiges can be functional, and speculative arguments against vestiges based upon their possible functions completely miss the point.

For more discussion of the vestigial concept, extensive modern and historical references concerning its definition (especially the allowance for functionality), see the Citing Scadding (1981) and Misunderstanding Vestigiality and 29+ Evidences for Macroevolution: Anatomical vestiges FAQs. The following discussion makes four main points: The human appendix may have bona fide functions, but this is currently controversial, undemonstrated in humans, and irrelevant as to whether the appendix is a true vestige or not. The appendix is a prime example of dysteleology (i.e. suboptimal structural design), a prediction of genetically gradual evolution . The appendix is a rudimentary tip of the caecum and is useless as a normal, cellulose-digesting caecum. Thus, the appendix is vestigial by both the evolutionary and non-evolutionary, typological definitions of vestigiality.

The vermiform appendix: background info Figure 1: The human vermiform appendix (image reproduced with modifications from Gray 1918 ) In humans, the vermiform appendix is a small, finger-sized structure, found at the end of our small caecum and located near the beginning of the large intestine ( Fawcett and Raviola 1994 , p. 636; Oxford Companion to the Body 2001 , pp. 42-43; Williams and Myers 1994 ). The adjective "vermiform" literally means "worm-like" and reflects the narrow, elongated shape of this intestinal appendage. The appendix is typically between two and eight inches long, but its length can vary from less than an inch (when present) to over a foot. The appendix is longest in childhood and gradually shrinks throughout adult life. The wall of the appendix is composed of all layers typical of the intestine, but it is thickened and contains a concentration of lymphoid tissue.

Similar to the tonsils, the lymphatic tissue in the appendix is typically in a constant state of chronic inflammation, and it is generally difficult to tell the difference between pathological disease and the "normal" condition ( Fawcett and Raviola 1994 , p. 636). The internal diameter of the appendix, when open, has been compared to the size of a matchstick. The small opening to the appendix eventually closes in most people by middle age. A vermiform appendix is not unique to humans. It is found in all the hominoid apes, including humans, chimpanzees, gorillas, orangutans, and gibbons, and it exists to varying degrees in several species of New World and Old World monkeys ( Fisher 2000 ; Hill 1974 ; Scott 1980 ). The caecum: a specialized herbivorous organ Our appendix is a developmental derivative and evolutionary vestige of the end of the much larger herbivorous caecum found in our primate ancestors ( Condon and Telford 1991 ; Williams and Myers 1994 , p. 9).

The word "caecum" actually means "blind" in Latin, reflecting the fact that the bottom of the caecum is a blind pouch (a dead-end or cul-de-sac). In most vertebrates, the caecum is a large, complex gastrointestinal organ, enriched in mucosal lymphatic tissue ( Berry 1900 ), and specialized for digestion of plants (see Figure 2 ; Kardong 2002 , pp. 510-515). The caecum varies in size among species, but in general the size of the caecum is proportional to the amount of plant matter in a given organism's diet. It is largest in obligate herbivores, animals whose diets consist entirely of plant matter. In many herbivorous mammals the caecum is as large as the rest of the intestines, and it may even be coiled and longer than the length of the entire organism (as in the koala). In herbivorous mammals, the caecum is essential for digestion of cellulose, a common plant molecule. The caecum houses specialized, symbiotic bacteria that secrete cellulase, an enzyme that digests cellulose.

Otherwise cellulose is impossible for mammals to digest. The structure of the caecum is specialized to increase the efficiency of cellulose fermentation. As a "side branch" from the gut it is able to house a large, dense, and permanent colony of specialized bacteria. Being a dead-end sac at the beginning of the large intestine, it allows more time for digesting food to reside in the gut and ferment more completely, before passing through the large intestine where the resulting nutrients are absorbed. However, even though humans are herbivorous, the small human caecum does not house significant quantities of cellulase-excreting bacteria, and we cannot digest more than but a few grams of cellulose per day ( Slavin, Brower, and Marlett 1980 ). Figure 2: Gastrointestinal tracts of various mammals. For each species, the stomach is shown at top, the small intestine at left, the caecum and associated appendix (if present) in magenta, and the large intestine at bottom right. Scale differs between species. Reproduced with modifications from Kardong 2002 , p. 511. Copyright © 2002 McGraw-Hill.

The human appendix is homologous to the end of the mammalian caecum In vertebrate comparative anatomy, it has long been known that the human appendix and the end of the mammalian caecum are structurally homologous ( Berry 1900 ; Fisher 2000 ; Hill 1974 ; Hyman 1979 , p. 412; Kardong 2002 , pp. 513-515; Kluge 1977 , p. 1977; Neal and Rand 1936 , p. 315; Romer and Parsons 1986 , p. 389; Royster 1927 , p. 27; Smith 1960 , p. 305; Weichert 1967 , p. 189; Wiedersheim 1886 , p. 236; Wolff 1991 , p. 384). Of course, the end of the caecum and the appendix can be homologous and have different functions.

Being the termination of the caecum, the human vermiform appendix is also a "blind pouch," and another name for the appendix is in fact the "true caecal apex" ( Berry 1900 ). Within the gastrointestinal tract of many vertebrates, mammals, and primates in particular, the termination of the caecum and the vermiform appendix share the same relative position ( Figure 2 ), both have a similar structure and form, both are blind sacs enriched with lymphatic tissue ( Berry 1900 ), both have a common developmental origin ( Condon and Telford 1991 ; Williams and Myers 1994 , p. 9), and, as discussed below, in the primates both are connected by an extensive series of intermediates. These observations firmly establish these structures as homologous by standard systematic criteria ( Kitching et al. 1998 , pp. 26-27; Remane 1952 ; Schuh 2000 , pp. 63-64; Rieppel 1988 , p. 202), a conclusion confirmed by cladistic systematic analysis ( Goodman et al.

1998 ; Shoshani 1996 ). A few other mammals appear to have a structure similar to the hominoid vermiform appendix, including the wombat, South American opossum (both marsupials), some rodents, and the rabbit. However, extensive comparative analysis has shown that the caecal appendixes of humans and these other mammals were derived from the caecum independently; these structures are not homologous as appendixes ( Shoshani and McKenna 1998 ). The relationship between these other caecal structures and the hominoid vermiform appendix is similar to the homology of bat and bird wings. The wings of bats and birds are homologous as modified forelimbs, yet they are not homologous as wings. Likewise, the appendixes of rabbits and humans are homologous as modified caeca, yet they are not homologous as appendixes. If the rabbit and human appendix have similar functions (which has never been experimentally demonstrated), they are the result of independent convergence of function and form ( Shoshani and McKenna 1998 ).

The many significant morphological, histological, and cellular differences between the rabbit and human appendixes (discussed below) all are consistent with a superficial convergent relationship. Structural intermediates of the appendix and caecum in primate phylogeny The primate family tree provides a rather complete set of intermediates between the states "large caecum/appendix absent" to "small caecum/appendix present". Many primates have both a caecum and an appendix, or a structure intermediate between the two. The anatomical definition of a vermiform appendix is a narrowed, thickened, lymphoid-rich caecal apex ( Fisher 2000 and references therein). As already mentioned, the hominoid apes all bear a vermiform appendix, but many non-anthropoid primates also have structures that fit the above definition to varying degrees. In fact, recent reevaluation of the anatomy of the primate caecum and appendix has highlighted the difficulties in determining exactly where the caecum ends and the appendix begins in different species ( Fisher 2000 ). This complication arises from the continuous, variable, and overlapping nature of caecal and appendicular tissues, both histologically and anatomically.

For example, in most primates the caecal apex is rich in lymphoid tissue and is thickened, but whether it is narrowed into a conical or "worm-like" structure is variable ( Fisher 2000 ). From systematic analysis of comparative anatomy, it is known that in primates a large caecum with a small or absent appendix is the ancestral, primitive state ( Goodman et al. 1998 ; Shoshani 1996 ). In general, the length of the caecum, relative to that of the colon, decreases as one traverses the primate phylogenetic tree from monkeys to humans. Concurrently, the size of the appendix increases. The appendix is mostly absent in prosimians and New World monkeys, yet they have a large caecum. In Old World monkeys the appendix is more recognizable, and it is well-developed in the anthropoid apes, which lack the large cellulose-fermenting caecum found in their ancestors and other primates ( Fisher 2000 ; Goodman et al. 1998 ; Hill 1974 ; Shoshani 1996 ; Scott 1980 ).

Possible function of the appendix "The appendix n, of the colon n m, is a part of the caecum and is capable of contracting and dilating so that excessive wind does not rupture the caecum." Leonardo da Vinci Earliest known drawing of the appendix, by Leonardo da Vinci. Throughout medical history many possible functions for the appendix have been offered, examined, and refuted, including exocrine, endocrine, and neuromuscular functions ( Williams and Myers 1994 , pp. 28-29). Today, a growing consensus of medical specialists holds that the most likely candidate for the function of the human appendix is as a part of the gastrointestinal immune system. Several reasonable arguments exist for suspecting that the appendix may have a function in immunity. Like the rest of the caecum in humans and other primates, the appendix is highly vascular, is lymphoid-rich, and produces immune system cells normally involved with the gut-associated lymphoid tissue (GALT) ( Fisher 2000 ; Nagler-Anderson 2001 ; Neiburger et al. 1976 ; Somekh et al. 2000 ; Spencer et al. 1985 ).

Animal models, such as the rabbit and mouse, indicate that the appendix is involved in mammalian mucosal immune function, particularly the B and T lymphocyte immune response ( Craig and Cebra 1975 ). Animal studies provide limited evidence that the appendix may function in proper development of the immune system in young juveniles ( Dasso and Howell 1997 ; Dasso et al. 2000 ; Pospisil and Mage 1998 ). However, contrary to what one is apt to read in anti-evolutionary literature, there is currently no evidence demonstrating that the appendix, as a separate organ, has a specific immune function in humans ( Judge and Lichtenstein 2001 ; Dasso et al. 2000 ; Williams and Myers 1994 , pp. 5, 26-29). To date, all experimental studies of the function of an appendix (other than routine human appendectomies) have been exclusively in rabbits and, to a lesser extent, rodents. Currently it is unclear whether the lymphoid tissue in the human appendix performs any specialized function apart from the much larger amount of lymphatic tissue already distributed throughout the gut.

Most importantly with regard to vestigiality, there is no evidence from any mammal suggesting that the hominoid vermiform appendix performs functions above and beyond those of the lymphoid-rich caeca of other primates and mammals that lack distinct appendixes. As mentioned above, important differences exist in nearly all respects between the human and rabbit appendixes ( Dasso et al. 2000 ; Williams and Myers 1994 , p. 57). The rabbit appendix, for instance, is very difficult to identify as separate from the rest of its voluminous caecum (see Figure 2 ). Unlike the human appendix, the rabbit's appendix is extremely large, relative to the colon, and is the seat of extensive cellulose degradation due to a specialized microflora. The large rabbit appendix houses half of its GALT lymphoid tissue, whereas the contribution of the human appendix to GALT is significantly less ( Dasso et al. 2000 ). In humans the vast majority of GALT tissue is found in hundreds of Peyer's patches coating the small intestine and in nearly 10,000 similar patches found in the large intestine.

Additionally, there are important differences in lymphoid follicular structure, in T-cell distribution, and in immunoglobulin density ( Dasso et al. 2000 ). Furthermore, from systematic analysis we know that the rabbit, rodent, and human appendixes are convergent as outgrowths and constrictions of the caecum ( Shoshani and McKenna 1998 ). It is thus very questionable to conclude from these animal studies that the human appendix has the same function as the other non-primate appendixes. Of course, over a century of medical evidence has firmly shown that the removal of the human appendix after infancy has no obvious ill effects (apart from surgical complications, Williams and Myers 1994 ). Earlier reports of an association between appendectomy and certain types of cancer were artifactual ( Andersen and Isager 1978 ; Gledovic and Radovanovic 1991 ; Mellemkjaer et al. 1998 ). In fact, congenital absence of the appendix also appears to have no discernable effect.

From investigative laparoscopies for suspected appendicitis, many people have been found who completely lack an appendix from birth, apparently without any physiological detriment ( Anyanwu 1994 ; Chevre et al. 2000 ; Collins 1955 ; Hei 2003 ; Host et al. 1972 ; Iuchtman 1993 ; Kalyshev et al. 1995 ; Manoil 1957 ; Pester 1965 ; Piquet et al. 1986 ; Ponomarenko and Novikova 1978 ; Rolff et al. 1992 ; Saave 1955 ; Shperber 1983 ; Tilson and Touloukian 1972 ; Williams and Myers 1994 , p. 22). In sum, an enormous amount of medical research has centered on the human appendix, but to date the specific function of the appendix, if any, is still unclear and controversial in human physiology ( Williams and Myers 1994 , pp. 5, 26-29).

The appendix is suboptimally designed The human appendix is notorious for the life-threatening complications it can cause. Deadly infection of the appendix at a young age is common, and the lifetime risk of acute appendicitis is 7% ( Addiss et al. 1990 ; Hardin 1999 ; Korner et al. 1997 ; Pieper and Kager 1982 ). The most common age for acute appendicitis is in prepubescent children, between 8 and 13 years of age. Before modern 20th-century surgical techniques were available, a case of acute appendicitis was usually fatal. Even today, appendicitis fatalities are significant ( Blomqvist et al. 2001 ; Luckmann 1989 ). The small entrance to this dead-end pocket makes the appendix difficult to clean out and prone to physical blockage, which ultimately is the cause of appendicitis ( Liu and McFadden 1997 ).

This peculiar structural layout is quite beneficial for a larger cellulose-fermenting caecum, but it is unclear why gut lymphoid tissue would need to be housed in a remote, dead-end tube with negligible surface area. In fact, 60% of appendicitis cases are due to lymphoid hyperplasia leading to occlusion of the interior of the appendix, indicating that the appendix is unusually prone to abnormal proliferation of its lymphoid tissue ( Liu and McFadden 1997 ). Such an occurrence would be much less problematic if the interior of the appendix were not so small, confined, and inaccessible from the rest of the gut. In many other primates and mammals, the GALT lymphoid tissue appears to function without difficulty in a much more open, bulbous caecum with ample surface area. Furthermore, there is mounting evidence that removing the appendix helps prevent ulcerative colitis, a nasty inflammatory disease of the colon ( Andersson et al. 2001 ; Buergel et al. 2002 ; Judge and Lichtenstein 2001 ; Koutroubakis and Vlachonikolis 2000 ; Koutroubakis et al.

2002 ; Naganuma 2001 ; Rutgeerts 1994 ). This evidence suggests that the appendix is actually maladaptive, and that the lymphoid tissue contained in the appendix is prone to chronic pathological inflammatory states. If the appendix does have an important function that we have yet to find, it is a leading candidate for the worst designed organ in the human body. How nice if the appendix would just degenerate away after it is no longer needed, so it could never get infected and kill us needlessly. Any biological structure that supposedly ensures our livelihood by its functions, yet paradoxically and unnecessarily kills a large fraction of its bearers prematurely, is poorly designed indeed. Why do some medical sources question the vestigiality of the appendix? The reasons for this are multiple, but they largely stem from the simple fact that most physicians are not trained in evolutionary biology. The erroneous "completely nonfunctional" definition of a vestige is primarily found in medical papers, textbooks, and dictionaries (e.g. Williams and Myers 1994 , p. ix). Using this incorrect and nonevolutionary definition, it is logical to conclude that a structure is not vestigial if its function is discovered.

For instance, based upon this incorrect definition, Williams and Myers 1994 incorrectly argue that an evolutionary vestige cannot be both a complex and a "regressive" structure (p. 27). Similarly, a modern version of Gray's Anatomy confusingly implies that the appendix cannot be both vestigial and specialized ( Williams and Warwick 1980 ). However, vestiges are very often complex or specialized structures, in fact overly complex for their functions, and prime examples are the wing of the ostrich and the eyes of blind cavefish. A vestige can be a complex structure, in an absolute sense, while simultaneously being rudimentary or degenerate relative to the same homologous structure in other organisms. Perhaps most important is the fact that a vestige can be identified only via comparative analysis. Physicians are experts on human anatomy and physiology, but rarely do discussions in medical publications consider phylogenetic and comparative issues. Medical articles that attempt to consider phylogenetics often provide a gross misconception of evolutionary fundamentals.

For instance, the most thorough and in depth source on the physiology of the human appendix, Williams and Myers 1994 , refers to how the appendix changes as "the primate scale is ascended" and to the "evolutionary scale" with humans at its end (pp. 26-27). These are long-refuted orthogenetic concepts which have been contradicted by basic evolutionary theory since Darwin. Scott 1980 similarly argues against vestigiality based upon orthogenetic concepts and a belief in evolutionary "progress." Fisher 2000 (p. 229) and Scott 1980 both incorrectly imply that a vestige cannot be a derived character, a curious assessment since a vestige must be a phylogenetically derived character by definition. How would we know if the appendix were not vestigial? Whether the appendix has a function of some sort or not has no direct bearing on whether it is a bona fide vestige.

However, at least three possible observations would help negate the conclusion that the human appendix is vestigial, using either the evolutionary or the typological definitions of vestigiality: (1) if the human appendix were actually as large and developed as, say, the caecum of a prosimian or New World monkey; (2) if the human appendix contributed significantly to cellulose fermentation and contained a large amount of cellulose-digesting bacteria; (3) if we could demonstrate via phylogenetic or systematic methods that the apex of the cellulose-fermenting caecum in other primates and the vermiform appendix were not structurally homologous as side branches from the intestine. An additional possible observation would contradict the conclusion of vestigiality by the evolutionary definition: (4) if phylogenetic methods indicated that no predicted ancestors of humans ever had a large, cellulose-fermenting caecum (i.e., that a large, cellulose-digesting caecum is actually a derived primate character, not a primitive one). All four of these potential observations are demonstrably false.

Additionally, each is based upon positive scientific evidence: (1) we can measure and quantitate the size of the appendix; (2) we can measure and quantitate the amount of cellulose digestion occurring in the appendix; (3) we can observe and compare the relative positions, underlying structures, forms, and development of the organs in the gastrointestinal tracts of various organisms; and (4) we can determine primitive and derived characters by independent phylogenetic analysis. Therefore, the conclusion of vestigiality is susceptible and open to scientific testing against empirical observation. As such the concept of vestigiality is not an "argument from ignorance." It is clearly scientific in nature, based completely upon positive evidence. Conclusion: The vermiform appendix is vestigial Currently, arguments against the vestigiality of the human vermiform appendix have been based upon misunderstandings of what constitutes a vestige and of how vestiges are identified. From an evolutionary perspective, the human appendix is a derivative of the end of the phylogenetically primitive herbivorous caecum found in our primate ancestors ( Goodman et al. 1998 ; Shoshani 1996 ).

The human appendix has lost a major and previously essential function, namely cellulose digestion. Though during primate evolution it has decreased in size to a mere rudiment, the appendix retains a structure that was originally specifically adapted for housing bacteria and extending the time course of digestion. For these reasons the human vermiform appendix is vestigial, regardless of whether or not the human appendix functions in the development of the immune system. From a nonevolutionary, typological perspective, the human appendix is homologous to the end of the physiologically important, large, cellulose-fermenting caeca of other mammals. Even though humans eat cellulose, the contribution to cellulose digestion by both the human caecum and its associated appendix is negligible. Regardless of whether one accepts evolutionary theory or not, the human appendix is a rudiment of the caecum that is useless as a normal mammalian, cellulose-digesting caecum. Thus, by all accounts the vermiform appendix remains a valid and classic example of a human vestige.

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Question: Which digestive organ is well-developed in grass-eating herbivores, but is only vestigial in humans?
Answer: Appendix